Is there less air or just less cold? Is the belt properly adjusted?

    Sounds like you're losing the engine vacuum that holds the vents open
(Volvo used vacuum solenoids to open/close the floor, windshield and
recirculation vents in the 240s).  Replace the check valve on the intake
(it's the one that sticks up from the intake runner #4, the one closest
to the firewall).  It might also be the vacuum bottle that it connects
to leaking or a hose being off or cracked on the interior ventilation
system, but those would also cause a rough/searching idle.  I've never
had to replace an interior component but have replaced the check valve
in many 240s.
     
    Bill

At the risk of sounding like a broken record, this sounds like a job for a
pro. I fear the compressor valves are failing, but R134a is spooky enough
that it could also be a symptom of slightly low charge. If it has been
evacuated and filled by weight you have the correct charge as you say,
otherwise probably not. A tech can also tell you by the pressure behavior if
the valves are leaky.

Or, as Bill says, it could also be vacuum problems. From my experience with
the vacuum problems in my '85 765T, if it is vacuum you should feel the air
getting cold within a couple seconds of letting off the throttle from
highway speeds. Charge problems shouldn't respond that quickly.

One more thing - a long shot but quick and cheap - try spraying the
condensor with a spray attachment on a garden hose. Dirt in there can do
weird things and it never hurts to make sure it's clean. (Been humbled by
that myself.)

Mike